It should be in the "I Know what I want" thread with Mom...Dunno, I went through several weeks of the same stuff happening over and over after I finished the Ann content (I had chosen the Lisa GF route). I'll wait for the next addon, I guess... Tried following the walkthrough back around page 175 and it didn't trigger the "private punishment" dialogue option for me.
I have found that sometimes changing status from Don't Spank Me to you can Spank me like everyone else sometimes opens up the conversation to private punishment after dinner.